Extruders are often used for conveying and processing substances. To separate gases during processing of plastics in an extruder, a gas outlet opening is provided in the housing of the process section of the extruder in which the solid or viscous plastic is processed. To prevent plastic from being discharged from the process section through said opening, the material retaining device is provided, also known as the “stuffer”. It has one or two screw shafts whose screw has a direction of thread over the total length by which it forces plastic entering said opening back into the process section, while gas passing through the screw can be drawn off.
As has turned out, however, plastic extruded with an extruder having such a retaining device shows a great number of black specks, which are extremely disturbing in particular in products made of transparent plastic, such as PET.
The problem of the invention is to provide an extruder having such a retaining device at the gas outlet opening which largely eliminates the formation of black specks.
This is obtained according to the invention by the extruder characterized in the claims. Advantageous embodiments of the invention are rendered in the subclaims.
As has turned out, the formation of black specks is due to the fact that the screw shaft and the inside wall of the retaining device have the same high temperature as the process section. In the known extruder, plastic residues adhering to the hot screw shaft or inside wall of the retaining device can thus decompose. Since the decomposition depends on the duration at which a plastic residue adheres to the hot metal surface of the screw or the inside wall of the retaining device, a plastic residue transported by the screw over its total length as far as the gas outlet opening in the intermediate housing of the material retaining device will naturally undergo particularly great decomposition. The solid products of decomposition are then, in the known extruder, conveyed by the material retaining device screw backfeeding over its full length back to the process section and mixed with the plastic in the process section to form the black specks in the extruded plastic product.
Starting out from this finding, the invention is based on giving only the portion of the material retaining device screw adjacent the process section a backfeeding design. The plastic is in contact with said portion only a short time. Moreover, in said portion the material is continually replaced on the hot screw surface and inside wall as in the process section. This prevents decomposition of the plastic and thus a formation of black specks in said portion.
In contrast, plastic residues passing through said portion of the material retaining device reach the screw portion with the opposite pitch according to the invention, i.e. they are no longer conveyed back to the process section but away therefrom and are thus separated from the process section. In this area they are thus exposed to the hot temperature of the screw and the inside wall of the intermediate housing for some time and are decomposed. The solid black decomposition specks can be drawn off together with the separated gas through the gas outlet opening in the intermediate housing, or an additional opening on the underside of the intermediate housing can be provided for the residues to drop through.
The process section of the inventive extruder can have only one screw shaft. However, there can also be provided two or more, in particular eight or more, axially parallel screw elements which can closely intermesh, the screw shafts being received and guided by axially parallel concave circular segments on the inside wall of the extruder housing. A plurality of said screw shafts can be disposed in a plane or form a ring, as described in EP 0 788 867 B1.
The material retaining device can likewise have only one screw shaft. However, it is preferable to provide two or more axially parallel screw shafts whose screws closely intermesh and which are in each case received and guided by axially parallel concave circular segments in the inside wall of the extruder housing.
It is possible to use corotating or counterrotating screws in the material retaining device. Since counterrotating screw shafts form an axially closed system unlike the axially open system of corotating screw shafts, i.e. counterrotating screw shafts cause forced conveyance, counterrotating screw shafts are generally preferred in the material retaining device since they prevent the occurrence of black specks even more rigorously.
Moreover, the self-cleaning of counterrotating screws is fundamentally better than of corotating screws, since with counterrotating screws their crests touch each other and lie against the inside wall of the intermediate housing, while corotating screws have a clearance between the crests of the screws and the inside wall. Wear can be combated by hardening the screws and the intermediate housing.
With counterrotating screw shafts it is furthermore possible to completely omit drive gears on the screw shafts if need be, since only one shaft needs to be driven which drives the other shaft by form-fitting engagement of the screws. This substantially simplifies the installation, assembly and maintenance of the screw shafts.
The screw shafts of the material retaining device are preferably formed in one piece with their drive, for example a drive gear. There is thus no coupling between screw shaft and drive gear as provided according to the prior art. The split gear box driving the screw shafts can thus be built small, light and economically.
So that no toxic gases can escape from the extruder unchecked, but also to prevent air from passing into the extruder, the intermediate housing with the material retaining device must be connected vacuumtight to the extruder housing and to the split gear box, whereby the split gear box must in turn be of vacuumtight form.
Since the split gear box contains bearings, seals and similar parts that can be damaged at the high temperature of the process section of for example approx. 250° C., it is advantageous to dispose heat insulating means, for example an insulating plate, between the intermediate housing with the material retaining device and the split gear box.
The inventive extruder is particularly suitable for processing plastics, such as PVC, PE and others. Since the formation of black specks is largely prevented, it is particularly suitable for producing transparent plastic products, in particular water-clear products, for example made of PET.